1. Field of the Invention
The present invention relates generally to a cemented carbide cutting tool, a cermet cutting tool or a ceramic cutting tool, and/or a coated cutting tool including the above cutting tool as a substrate, which are suitable for use in an indexable insert, and, more particularly, to a method of increasing the toughness and chipping resistance of a cutting tool.
2. Description of the Related Art
In general, to increase the effective lifetime of a cemented carbide cutting tool, the surface of a cemented carbide alloy substrate is coated with a hard ceramic film formed of titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), or alumina (Al2O3), by means of chemical vapor deposition (CVD), physical-chemical vapor deposition (PCVD) or physical vapor deposition (PVD).
As the coated cutting tool deposited with the Al2O3 film by means of CVD, a cutting tool composed of a TiC film coated with a 0.5-1.0 μm thick Al2O3 film was first reported in 1973. Although the cutting tool including the TiC film coated with the Al2O3 film has slightly lower toughness than a cutting tool including a mono-layered TiC film, it has drastically increased abrasion resistance.
Therefore, to increase the toughness of the coated cutting tool, a TiCN film, coated with an organic CN compound precursor (acetonitrile, CH3CN, etc.) at 800-900° C. by means of moderate temperature-CVD (MT-CVD), is used. That is, the TiCN film, which has been typically deposited using a gas material, such as TiCl4, CH4, N2 or H2, at about 1000-1050° C. by means of high temperature-CVD (HT-CVD), can be deposited by means of MT-CVD at 800-900° C. using gas, such as TiCl4, CH3CN, N2 or H2. Hence, although the TiCN film coated by means of MT-CVD has lower film hardness than the TiC film, it retains film hardness sufficient to increase the abrasion resistance of the cemented carbide alloy coated therewith. Further, since the TiCN film has a columnar crystal structure, the above film per se has high toughness.
EP 408,535 discloses that, as results of phase control studies for an Al2O3 film having high oxidation resistance, an α-alumina (α-Al2O3) film and a κ-alumina (κ-Al2O3) film are suitable for processing cast iron and steel, respectively. Thereby, control techniques for the Al2O3 film have been rapidly developed and commercialized. In the case of α-Al2O3, it is advantageous in that because it is only a stable phase among the phases of Al2O3, it is not phase transformed during cutting. In addition, the above Al2O3 film having high hardness can exhibit superb cutting performance when cutting cast iron at high speed. In the case of κ-Al2O3, since it has lower heat conductivity than α-Al2O3, it manifests high abrasion resistance when cutting steel, which generates lots of heat.
Further, the coating on the coated cutting tool intended to enhance cutting performance may chip away or flake away. The flaking, acting to increase the abrasion of a rake face of the tool, or chipping is caused by low adhesion between the substrate and the film or cut workpiece chips fused on a cutting edge of the cutting tool.
The coated cutting tool manufactured by means of CVD has high abrasion resistance. However, since residual tensile stress attributed to the difference in heat expansion coefficients between the substrate and the film is present on the film, the coated tool has low toughness. Moreover, the coated tool is disadvantageous because chipping, which causes the film to partially chip away when used for cutting, takes place or the film may be wholly detached from the substrate, thus drastically decreasing the lifetime of the tool.
To solve the above problems, WO 9,923,275 discloses a coated cemented carbide body which is subjected to wet blasting at a pressure of 2-6 bar using 30 μm sized Al2O3 particles. However, the above invention is limitedly applied to a specific layer structure where an Al2O3 layer is deposited below an outer layer formed of TiN having a thickness of at least 4 μm or a multi-layer film formed of TiN/TiC having a thickness of 4-16 μm. This is because the outer layer formed of TiN weakly adhering to the Al2O3 layer is easily removed upon wet blasting. In addition, the wet blasting treatment using Al2O3 particles, which is used to decrease the surface roughness of the tool, hardly affects the change of residual stress, compared to dry blasting.
U.S. Pat. No. 5,635,247 discloses a method of producing an alumina coated cemented carbide body, comprising wet blasting a surface of a κ-Al2O3 coating film deposited on a cemented carbide substrate using 100 μm sized Al2O3 particles under 2-6 bar, and then heat treating the surface of κ-Al2O3 at 900-1100° C. for 0.3-10 hours to cause phase transformation of the κ-Al2O3 into α-Al2O3, thus increasing the chipping resistance of the film. However, the above invention is disadvantageous because the coating film subjected to both wet blasting and heat treatment has surface roughness inferior to the coating film subjected to only wet blasting.
In addition, U.S. Pat. Nos. 5,597,272 and 5,776,588 disclose a coated hard alloy tool, in which the top several layers of a multi-layer ceramic coating film of the tool are partially or completely missing along a cutting edge of the tool. As such, at least one oxide layer is included in the missing layers, and the residual stress on the film exposed along the cutting edge is changed to +98˜−49 Mpa, and the surface roughness thereof is controlled in the range of 0.05 μm or less. However, the change of the residual stress on the film exposed along the cutting edge is too low to increase the toughness of the tool, and also, limitations are imposed on the uniform removal of only the top several layers of the cutting edge.
US Serial No. 2003/0104254 discloses a method of decreasing residual stress present on a multi-layered coating film using dry blasting to increase toughness, in which the multi-layered coating film formed of TiCN/Al2O3/ZrCN on a substrate has residual stress of +0.8/+0.4/−1.0 GPa. However, after the blasting using various particles, the residual stress of the coating film is changed as follows, that is, an outer layer formed of ZrCN of the coating film has residual stress of −4.0˜−8.0 GPa, the Al2O3 layer has residual stress of −2.6˜−4.3 GPa, and the TiCN layer has residual stress of −0.5˜−1.7 GPa. From this, it can be seen that the tensile stress is converted into the compressive stress, resulting in increased compressive stress. However, the coated cutting tool subjected to dry blasting has surface roughness inferior to cutting tools subjected to wet blasting, and also, the residual stress present on the coating film thereof is extremely high, thus the coated cutting tool is easily breakable.